Certain tumors, benign, premalignant, and malignant, are known to have genetic components. Some of these tumors are caused by mutations or inactivation of “tumor suppressor” genes. In normal cells, the tumor suppressor genes are involved in the regulation of cell growth and proliferation and in the control of cellular aging, anchorage dependence and apoptosis. When the tumor suppressor genes are mutated or inactivated, cells are transformed and become immortalized or tumorigenic. These transformed cells can be reverted back to the normal phenotype (i.e., the cell growth rate is suppressed) by introducing the wildtype suppressor genes.
The first tumor suppressor gene identified was the nuclear phosphoprotein, retinoblastoma gene (Rb). Retinoblastoma is a malignant tumor of the sensory layer of the retina, and often occurs bilaterally during childhood. Retinoblastoma exhibits a familial tendency, but it can be acquired. Mutations in the Rb gene and inactivation of its product have been shown to be involved in other tumors, such as bladder, breast, small cell lung carcinomas, osteosarcomas, and soft tissue sarcomas. It was demonstrated that reconstitution of Rb-deficient tumor cells with the wildtype Rb leads to the suppression of growth rate or tumorigenicity (Huang et al., Science 242:1563-1566 (1988)). This result provides direct evidence that Rb protein is a tumor suppressor.
Another well-characterized tumor suppressor is the gene for the nuclear phosphoprotein, p53. More than half of all human cancers are associated with mutations in the tumor suppressor gene p53 (see, e.g., Hollstein et al., Science 253:49-53 (1991); Caron de Fronmentel & Soussi, Genes Chromosom. Cancer 4: 1-15; Harris & Hollstein, N. Engl. J. Med. 329:1318-1327 (1993); Greenblatt et al., Cancer Res. 54:4855-4878 (1994)). Mutations in p53 often appear to be a critical step in the pathogenesis and progression of tumors. For example, missense mutations of p53 occur in tumors of the colon, lung, breast, ovary, bladder, and several other organs. Alternatively, inactivation of the wildtype p53 proteins in cells can cause tumors. For example, certain strains of human papillomavirus (HPV) are known to interfere with the p53 protein function, because the virus produces a protein, E6, which promotes the degradation of the p53 protein.
Recently, another tumor suppressor gene, p33ING1, has been identified. p33ING1 directly cooperates with tumor suppressor gene p53 in growth regulation (Garkavtsev et al., Nature Genetics 14:415-420 (1996); Garkavtsev et al., Nature 391:295-298 (1998); U.S. Pat. No. 5,986,078, all of which are herein incorporated by reference). Neither of p53 or p33ING1 can alone cause growth inhibition when the other one is suppressed (Garkavtsev et al. (1998), supra). According to immunoprecipitation studies, p33ING1 proteins modulate the p53 activity through physical interaction. It has been also reported that some neuroblastoma cells have a mutation of the p33ING1 gene, and some breast cancer cell lines exhibit reduced expression of p33ING1 (Garkavtsev et al. (1996), supra).
Cancer remains a major public concern. Although epidemiological and cytogenetic studies demonstrated that a number of recessive genetic mutations are involved in various cancers, only a limited number of tumor suppressors have been identified. Therefore, there is a need to identify and isolate other tumor suppressor genes. The identification and isolation of new tumor suppressor genes would assist the diagnosis, prevention, and treatment of tumors and cancers.